Organic dye molecules capable of emitting fluorescence are often able, in suitable solvents, to produce coherent emissions as a result of optical excitation and are thus generally usable as laser dyes. A detailed overview of organic laser dyes of .lambda..sub.max &gt;400 nm is given by K. H. Drexhage "Structure and Properties of Laser Dyes", pp. 144 to 193 in "Dye Lasers", Topics in Applied Physics, Volume 1, Publisher F. P. Schafer, Springer Verlag (Berlin-Heidelberg--New York), 1977.
Optical excitation is effected with the aid of light sources, such as gas lasers ("pump lasers"). Examples of these are nitrogen, argon and krypton ion lasers, and excimer lasers. The dissolved organic molecules of the laser dye are brought from their ground state into an electronically excited state. If the number of laser dye molecules in the electronically excited state exceeds the number of molecules in the ground state (population inversion), stimulated emission of light can take place. This stimulated emission is started in an optical resonator where a quartz cell contains the solution of the laser dye. Circulating the dye-solution in a closed circuit avoids local overheating which would lead to optical inhomogeneities (schlieren formation). The closed unit of the optical system including mirrors, prisms, reflection grating and dye-solution cell is called a dye laser. Within the range of fluorescence emission of the laser dye, any desired wavelength can be coupled out of such a dye laser.
The great advantage of dye lasers compared to gas lasers is that it is possible to select tuned wavelengths for many different applications over a wide range of the fluorescence spectrum of the organic dye. Fields of use for such variable frequency lasers are, inter alia, chemical analytics, high resolution spectroscopy, fluorescence spectroscopy, photoionization spectroscopy, isotope separation, and etc.
The laser dyes known today cover a range from the near ultraviolet (about 320 nm) to the near infrared (about 1000 nm). In the UV range, particularly in the range below 450 nm, there are only a few photostable laser dyes. Since the energies radiated in the UV range by the pump laser lie in the range of the binding energies of organic molecules, the stimulated emission competes with the photochemical decomposition (e.g. with the decay) of the laser dye. Due to the very high energy density of the pump laser, the quantum yield for the photochemical decomposition of the laser dye must lie below 10.sup.-6. The photostability of the laser dye is significant particularly for long-term tests. Thus, for example, although compounds from the class of coumarins are excellent laser dyes which emit in the blue range, they exhibit only poor photostability [B. H. Winters, H. I. Mandelberg and W. B. Bohr, Appl. Phys. Letters 25, 723, (1974)]. Similar conditions exist with the patented bisstyryl-biphenyl compounds (German Pat. No. 2,700,292) and with some laser dyes belonging to the oxazol and oxadiazole series which are used also as organic scintillators [V. S. Antonov and K. L. Hohla, Appl. Phys. Volume 32, 9 (1983)].
In experiments to obtain a shorter wave laser emission than 323 nm, which was accomplished in a 1.times.10.sup.-3 molar paraterphenyl solution in cyclohexane, substituted paraterphenyls were found (DE-OS No. 3,007,234). The substituted p-terphenyls mentioned in Table 2 of this publication have laser dye tuning ranges (.DELTA..lambda.) which lie between 311.2 and 360.5 nm. P-terphenyl itself has a tuning range between 321.8 and 365.5 nm.
Fluorescence dyes that can be derived from p-quater-phenyl and are able to support 2 to 4 sulfonic acid ester substituents and further substituents, such as, for example, the dye known by the abbreviated form "Polyphenyl 1" ##STR1## and their use as laser dyes are known from DE-OS No. 2,938,132.